A Hopf bundle over a quantum four-sphere from the symplectic group

We construct a quantum version of the SU(2) Hopf bundle $S^7\to S^4$. The quantum sphere $S_q^7$ arises from the symplectic group $S{p}_q(2)$ and a quantum 4-sphere $S_q^4$ is obtained via a suitable self-adjoint idempotent $p$ whose entries generate the algebra $A(S_q^4)$ of polynomial functions over it. This projection determines a deformation of an (anti-)instanton bundle over the classical sphere $S^4$. We compute the fundamental $K$-homology class of $S_q^4$ and pair it with the class of $p$ in the $K$-theory getting the value -1 for the topological charge. There is a right coaction of $S{U}_q(2)$ on $S_q^7$ such that the algebra $A(S_q^7)$ is a non trivial quantum principal bundle over $A(S_q^4)$ with structure quantum group $A(S{U}_q(2))$.